Looking at What We Can’t See:Pulsar Radio Observations

ST 562 Radio Astronomy For Teachers

By: Cecilia Huang and Joleen Welborn

The Tools

• SRT: Single Radio Telescope

One telescope detects and records radio waves at differentfrequencies. We took observations at 1420.4 MHz, the emission frequency of neutralhydrogen.

• N2I2: InterferometerTwo 3.05 m diameter radio telescopes situated 24 m apart will detect at different frequenciesas well, but can also be used to calculate RA and Declination. Operates in 3 possible modes: tracking, meridian and non-meridian (drift), and as a single dish. Together, the telescopes act as if they were a singletelescope with higher resolution.

http://www.cassicorp.com

http://www.nrao.edu/epo/amateur/N2I2.pdf

N2I2 Drift Scan of Sun

• We took a reading using the N2I2 interferometer in drift mode. We lined the telescope up and let the object drift into the beam. The fringes of the sun are predictably regular and calculations of the fringe period match the theoretical. We did this to make sure the equipment is functioning well.

Sun Scan

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

1 51 101 151 201 251 301 351

Time (Sec)

Pow

er

Calculating Fringe Period

Sun Scan11:30 a.m. July 24, 2006

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

1 51 101 151 201 251 301 351

Time (Sec)

Po

wer~3 fringes over 100 seconds

Divide number of fringes over that period of time. This gives the fringe period, or the number of fringes per second. The number we get can be plugged into the following equation determine what the RA and DEC is.

t = λ / by ωecos δ

If this matches the actual position of the sun, we can conclude the equipment is working properly.

Divide number of fringes over that period of time. This gives the fringe period, or the number of fringes per second. The number we get can be plugged into the following equation determine what the RA and DEC is.

t = λ / by ωecos δ

If this matches the actual position of the sun, we can conclude the equipment is working properly.

Choosing the Project

• Because of the fascinating nature of pulsars, we thought it would be interesting to observe one with one of these radio telescopes.

• We chose the pulsar in the Crab Nebula, PSR 0531+21

What is a Pulsar?

• Discovered in the 1960’s by Dr. Jocelyn Bell who, as a grad-student, was searching radio strip charts for something new.

• Neutron star• Very, very dense• Spins really fast• Emits high energy particles like x-rays• Magnetic fields are intense• “Pulses” over regular periods of time with

electromagnetic radiation.

Diagram of a Pulsar

Image from http://glast.gsfc.nasa.gov/public/science/pulsars.html

X-Ray Image of the Pulsar in the Crab Nebula

Image from http://glast.gsfc.nasa.gov/public/science/pulsars.html

What We Expected

At the beginning of the course, we weren’t quite sure what to expect, so we performed the “shotgun approach” when choosing our observations, hoping to find something thatwould tell us a bit about pulsars.

We expected that:

• Drift Scans of known pulsars with SRT would show obvious spikes at predictable or regular times.

• N2I2 would show fringes with which we could run calculations that would determine RA and Dec or compare with theoretical fringe periods.

Drift Scan of Crab Nebula using N2I2

Crab Nebula Pulsar N2I2 Drift Scan

2.7

2.75

2.8

2.85

2.9

2.95

3

3.05

3.1

3.15

3.2

1 51 101 151 201 251 301 351

Time (sec)

Inte

nsi

ty

Second Reading, 30 minutes later

Crab Nebula #2

2.32

2.34

2.36

2.38

2.4

2.42

2.44

2.46

2.48

2.5

2.52

1 51 101 151 201 251 301 351

Time

Inte

nsi

ty

Calculations

• We determined the fringe period of both graphs by dividing the average number of fringes by the period of time that went by.

• We found that not only were the graphs very different, so were the fringe periods.

Crab Scan I: 23.33 seconds per fringeCrab Scan II: 15.22 seconds per fringe

Calculating the Theoretical Fringe Period

t = λ / by ωecos δ• t is the fringe period in seconds

• λ is the wavelength of the observation, in this case, 20 cm or 0.2 m

• by is the baseline distance between telescopes, 24 m

• ωe is the equatorial rotation of Earth which is about 7.29 x 10-5 radians per second

• cos δ is the cosine function of the declination angle

Using this calculation, the theoretical fringe period should be near

124.2 seconds per fringe. Unfortunately, neither of our observations

came anywhere near the theoretical.

Speculated Possibilities for This Outcome

• The N2I2 has fairly accurately detected this pulsar before. Perhaps the N2I2 has lost some of its sensitivity since the hail storm.

• Observation point too close to the sun and we got a lobe.

• Pulsars are just REALLY difficult to detect using interferometry.

What does the SRT tell us?

Our next observation was with the

SRT. We wanted to see if there were

going to be any regularly spaced

“pulses” from the Crab Nebula on the

graph.

SRT Scan of PSR 0531-21Drift Scan of PSR 0531 -21

900

920

940

960

980

1000

1020

1040

0 300 600 900 1200 1500 1800 2100 2400 2700 3000

Time Stamp

Inte

nsi

ty

Pulse Frequency

• To get the pulse frequency, we counted the peaks and divided the number over the amount of time passed. We tried to be as discriminating as possible,but it was rather difficult.

• # of “peaks” between 58 and 65.• Time of observation ~ 19 minutes, or 1140

seconds.• 58/1140, 65/1140 = 0.051, 0.057 seconds

between pulses, or 19.7,17.53 pulses per second.

• Compare to the actual period pulse of the Crab Nebula: which is 0.033 seconds or about 29 times per second.

YAY!!!

• That’s pretty darn close!

• However – We may have a better number if we took a longer reading and there was no lag in the data stream between the AOC and the VLA. PLUS, there may be a sensitivity issue.

Future Observations

• I don’t think we should abandon the pulsar observation with N2I2. I believe we can get close to the theoretical fringe period by taking several more observations and averaging them out somehow.

• Observe during a time when the sun’s declination is not so close to the pulsar.

• Look at other known pulsars, such as PSR 0329+54

References:

• Danielle’s interferometer design paper: http://www.nrao.edu/epo/amateur/N2I2.pdf

• Instructions on how to use the SRT: http://www.astro.cf.ac.uk/observatory/radioman.html

• Pulsar explanations, diagrams, and images: http://glast.gsfc.nasa.gov/public/science/pulsars.html

• Coordinate System of RA and Dec: http://www.go-astronomy.com/articles/coordinate-system.htm

• Amateur Radio Astronomy Projects: http://www.radiosky.com/rspplsr.html• Messier Object Help:

http://longmontastro.org/albers/las/messier/mess_02_05.pdf• Lyne, A.G. and Graham-Smith, F., “Pulsar Astronomy”; Cambridge

Astrophysics Series, 1990 (ISBN:0-521-32681-8)